ICE during cross-compilation and const eval

[alexcrichton]

Looks like today's nightly unfortunately contains a regression from #46882 (confirmed locally) which causes rustc to ICE specifically when cross-compiling

#![crate_type = "rlib"]
pub fn foo() {
    std::env::consts::ARCH;
}

$ RUST_BACKTRACE=1 rustc src/lib.rs --target i686-unknown-linux-gnu
warning: path statement with no effect
 --> src/lib.rs:3:5
  |
3 |     std::env::consts::ARCH;
  |     ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(path_statements)] on by default

thread 'rustc' panicked at 'slice index starts at 4294967295 but ends at 8288060', libcore/slice/mod.rs:791:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:59
             at libstd/panicking.rs:207
   3: std::panicking::default_hook
             at libstd/panicking.rs:223
   4: core::ops::function::Fn::call
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:403
   6: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:349
   7: rust_begin_unwind
             at libstd/panicking.rs:325
   8: core::panicking::panic_fmt
             at libcore/panicking.rs:72
   9: core::slice::slice_index_order_fail
             at libcore/slice/mod.rs:791
  10: <serialize::opaque::Decoder<'a> as serialize::serialize::Decoder>::read_usize
  11: <rustc_metadata::decoder::DecodeContext<'a, 'tcx> as serialize::serialize::SpecializedDecoder<rustc::mir::interpret::AllocId>>::specialized_decode
  12: <rustc_metadata::decoder::DecodeContext<'a, 'tcx> as serialize::serialize::SpecializedDecoder<rustc::mir::interpret::AllocId>>::specialized_decode
  13: <rustc::mir::interpret::MemoryPointer as serialize::serialize::Decodable>::decode::{{closure}}
  14: serialize::serialize::Decoder::read_enum_variant
  15: serialize::serialize::Decoder::read_enum_variant
  16: serialize::serialize::Decoder::read_enum_variant
  17: <rustc::mir::Operand<'tcx> as serialize::serialize::Decodable>::decode
  18: serialize::serialize::Decoder::read_seq
  19: <rustc::mir::Mir<'tcx> as serialize::serialize::Decodable>::decode::{{closure}}
  20: rustc_metadata::decoder::<impl rustc_metadata::cstore::CrateMetadata>::maybe_get_optimized_mir
  21: rustc_metadata::cstore_impl::provide_extern::optimized_mir
  22: rustc::dep_graph::graph::DepGraph::with_task_impl
  23: rustc::ty::maps::<impl rustc::ty::maps::queries::optimized_mir<'tcx>>::force
  24: rustc::ty::maps::<impl rustc::ty::maps::queries::optimized_mir<'tcx>>::try_get
  25: rustc::ty::maps::TyCtxtAt::optimized_mir
  26: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::maybe_optimized_mir
  27: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::load_mir
  28: rustc_mir::interpret::const_eval::eval_body_and_ecx::{{closure}}
  29: rustc_mir::interpret::const_eval::eval_body_and_ecx
  30: rustc_mir::interpret::const_eval::const_eval_provider
  31: rustc::dep_graph::graph::DepGraph::with_task_impl
  32: rustc::ty::maps::<impl rustc::ty::maps::queries::const_eval<'tcx>>::force
  33: rustc::ty::maps::<impl rustc::ty::maps::queries::const_eval<'tcx>>::try_get
  34: rustc::ty::maps::TyCtxtAt::const_eval
  35: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::const_eval
  36: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::read_global_as_value
  37: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::eval_operand
  38: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::eval_rvalue_into_place
  39: rustc_mir::interpret::const_eval::eval_body_and_ecx::{{closure}}
  40: rustc_mir::interpret::const_eval::eval_body_and_ecx
  41: rustc_mir::interpret::const_eval::const_eval_provider
  42: rustc::dep_graph::graph::DepGraph::with_task_impl
  43: rustc::ty::maps::<impl rustc::ty::maps::queries::const_eval<'tcx>>::force
  44: rustc::ty::maps::<impl rustc::ty::maps::queries::const_eval<'tcx>>::try_get
  45: rustc::ty::maps::TyCtxtAt::const_eval
  46: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::const_eval
  47: rustc_mir::transform::const_prop::ConstPropagator::eval_constant
  48: <rustc_mir::transform::const_prop::ConstPropagator<'b, 'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_statement
  49: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  50: rustc_mir::transform::optimized_mir::{{closure}}
  51: rustc_mir::transform::optimized_mir
  52: rustc::dep_graph::graph::DepGraph::with_task_impl
  53: rustc::ty::maps::<impl rustc::ty::maps::queries::optimized_mir<'tcx>>::force
  54: rustc::ty::maps::<impl rustc::ty::maps::queries::optimized_mir<'tcx>>::try_get
  55: rustc::ty::maps::TyCtxtAt::optimized_mir
  56: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::instance_mir
  57: rustc_mir::monomorphize::collector::collect_items_rec
  58: rustc_mir::monomorphize::collector::collect_crate_mono_items
  59: rustc_trans::base::collect_and_partition_translation_items
  60: rustc::dep_graph::graph::DepGraph::with_task_impl
  61: rustc::ty::maps::<impl rustc::ty::maps::queries::collect_and_partition_translation_items<'tcx>>::force
  62: rustc::ty::maps::<impl rustc::ty::maps::queries::collect_and_partition_translation_items<'tcx>>::try_get
  63: rustc::ty::maps::TyCtxtAt::collect_and_partition_translation_items
  64: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::collect_and_partition_translation_items
  65: rustc_trans::base::trans_crate
  66: <rustc_trans::LlvmTransCrate as rustc_trans_utils::trans_crate::TransCrate>::trans_crate
  67: rustc_driver::driver::phase_4_translate_to_llvm
  68: rustc_driver::driver::compile_input::{{closure}}
  69: rustc::ty::context::TyCtxt::create_and_enter
  70: rustc_driver::driver::compile_input
  71: rustc_driver::run_compiler

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.26.0-nightly (c90f68224 2018-03-08) running on x86_64-unknown-linux-gnu

cc @oli-obk

[oli-obk]

OK this is weird. It looks like the metadata contains u32::max_value() - 1, even though it should be u64::max_value() - 1, because usize on the host machine is u64.

See https://github.com/rust-lang/rust/blob/master/src/librustc_metadata/decoder.rs#L285 for the decoding and https://github.com/rust-lang/rust/blob/master/src/librustc_metadata/encoder.rs#L217 for the encoding.

The target bitwidth should not influence usize serialization to metadata, right?

Or is the libstd built on i686 and now read on x86_64? That would mean we should investigate all uses of usize/isize serialization in metadata, because they all may have similar problems.

I'll fix it on Friday, until then, anyone can fix this by making 0 and 1 reserved indices instead of usize::max_value and its predecessor

[oli-obk]

Or, just use u32 instead of usize, which means that 64 bit hosts can have a maximum of 2^32 allocations that go into metadata. We can then revisit when it becomes an issue because we run out of indices

[oli-obk]

Ah no, I misunderstood, there's no metadata file that can be bigger than 4 gigabytes. We should probably forbid encoding usize to metadata with a specialized encoder that panics. Because any metadata file that's bigger than 4GB will cause similar errors to pop up

[nikomatsakis]

triage: P-high